1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus which records information onto or reproduces information from a recording medium, such as an optical disk, by focusing a laser beam emitted by a semiconductor laser, on a recording layer of the recording medium.
2. Description of the Related Art
In these years, there are various optical read-only recording media, such as CD (compact disk), CD-ROM (compact disk read-only memory), DVD (digital versatile disk), etc., and optical reproducing systems, or optical disk players, which reproduce information from these recording media, are put into practical use.
In addition, the read-only recording media have their rewritable equivalents, including optical write-once read-many recording media (such as CD-R), magneto-optical recording media (such as MO), and phase-change recording media (such as DVD-rewritable). The special attention is given to the phase-change recording media (typically, DVD-rewritable disks) as mass-storage recording media in the next generation, and optical recording/reproducing systems, or optical disk drives, which record data onto and reproduce data from the phase-change media are proceeding towards practical applications.
The phase-change recording media utilize a process called the phase change technology to write and erase data. In this process, data is written to the phase-change recording disk by focusing a high-intensity laser beam on a recording layer of a phase-change material embedded in the substrate of the disk. In its original state of the phase-change material, the recording layer has a crystalline structure. The laser beam selectively heats areas of the surface portion of the disk to a high temperature. Where the beam strikes, the heat melts the crystals to a non-crystalline, or amorphous phase. These areas reflect less light than the unchanged area surrounding them.
When a weaker laser beam, used to read data from the disk, strikes the amorphous area, the beam is scattered and not picked up by the light-sensitive diode in the read head of the disk drive. With the lower reflectance, these areas become marks, representing “1”s. Areas that are not heated are more reflective areas, representing “0”s. When the read laser beam strikes the areas, it is reflected directly to the light-sensitive diode of the read head, creating an electrical current that is sent to the controller in the disk drive. The controller interprets the pattern of electrical pulses, decodes the data that they represent, checks the data for error, and sends the data to a computer.
To erase data or to change a mark back to crystalline phase, the disk drives use a lower-energy laser beam to heat marked areas to a relatively low temperature. This amount of heat is below the melting point of the phase-change material, but it still loosens up the phase change recording media so that it can recrystallize to the original state.
Apart from the magneto-optical media, the phase-change recording media do not require the application of an external magnetic field to the recording media, and it is possible to read, write, and erase data with respect to the phase-change disk by only focusing a laser beam emitted by a laser diode (LD), onto the recording layer of the disk.
If an optical recording/reproducing apparatus uses a single-pulse laser driving waveform when recording data onto the phase-change recording medium, the heating or the cooling of the recording layer of the disk is often likely to be insufficient for the formation of non-crystalline phase or crystalline phase in the recording layer, which will produce an undesired pattern or an error caused when reproducing the recorded data from the recording medium. In order to eliminate the above problem and reliably reproduce the recorded data from the recording medium without producing the undesired pattern, the optical recording/reproducing apparatus is required to use a multi-pulse laser driving waveform when recording data onto the phase-change recording medium.
A mark portion of the multi-pulse laser driving waveform includes a head-end high-level signal portion, a plurality of subsequent high-level signal portions, and a plurality of intermediate low-level signal portions between the high-level signal portions. The head-end and subsequent high-level of the drive current correspond to a peak power “Pw” for the laser beam of the laser diode to heat the recording layer of the disk to a high temperature above the melting point of the phase-change material. The intermediate low level of the drive current corresponds to a bottom power “Pb” for the laser beam of the laser diode to cool the recording layer of the disk. Suppose that a read-process power for the laser beam of the laser diode during the reading process is represented by “Pr”. The peak power “Pw”, the bottom power “Pb” and the read-process power “Pr” are predetermined such that they satisfy the following conditions.Pw>Pb=Pr  (1)
A space portion of the multi-pulse laser driving waveform includes a single middle-level signal portion. The middle level of the drive current corresponds to an erase power “Pe” for the laser beam of the laser diode to erase the data in the recording layer of the disk. The erase power “Pe” is predetermined such that it satisfies the following conditions.Pw>Pe>Pb  (2)
When the optical recording/reproducing apparatus uses the above-described multi-pulse laser driving waveform when recording data onto the phase-change recording medium, it is possible to eliminate the problem of the single-pulse laser driving waveform and reliably reproduce the recorded data from the recording medium without producing the undesired pattern.
Further, when recording data onto the phase-change recording medium, the optical recording/reproducing apparatus is required to properly carry out the laser power control.
Generally, the laser diodes have the light vs. current characteristics. The light output is relatively small until the current reaches a threshold current. Thereafter the optical intensity rises approximately linearly with increasing current. For digital modulation, the current to the laser diodes switches between two levels, the 0 level current being near the threshold current and the 1 level current being higher. The problems associated with typical laser diodes are that the characteristic curve bends over at high current and tends to shift and bend to the right with increasing temperature.
A method for stabilizing the optical power of a laser diode is the automatic power control (APC). The optical recording/reproducing apparatus usually executes the APC process to stabilize the optical power of the laser diode.
When the APC process is performed, part of the laser beam emitted by the laser diode is received at a photodetector (PD), and the photodetector outputs a monitoring current the amplitude of which is proportional to the optical power of the laser beam. By utilizing the monitoring current output by the photodetector, the drive current to the laser diode is controlled in the APC process.
When the APC process is performed for the reading of the phase-change recording medium, a high-frequency current is superimposed on the drive current to the laser diode so as to reduce the noises. The drive current can be assumed as being a constant current. By providing a feedback loop having frequencies that are within a relatively low frequency range, the APC process can be performed.
When the APC process is performed for the writing of the optical recording media, the recording power of the laser diode is quickly shifted between the different levels in order for the formation of marks and spaces in the recording layer of the disk. Some corrective measures must be taken for the APC process.
For CD and DVD media, the requirement that a digital sum value (DSV) of the recording data should be set to zero is met. By providing a feedback loop with the limited bandwidth that is within a relatively low frequency range, the APC process for the writing of the recording media, which is essentially the same as the APC process for the reading of the recording media, can be performed with a simple configuration of the optical recording/reproducing apparatus. However, it is difficult to provide accurate power control for the optical power of the laser diode during the writing.
For the CD-R media, the write pulse strategy shown in FIG. 11 is used by a conventional optical recording/reproducing apparatus. The writing of the CD-R media is performed with the write pulse strategy shown in FIG. 11. When a mark or a space having a maximum length of 11T (T indicates a unit length corresponding to a period of a channel clock) is recorded on the disk, the output power of the laser diode corresponding to each of the mark and the space is sampled and held by the sample/hold circuit. Even when the speed of the disk rotation is set at the quadruple speed, the required bandwidth of the photodetector and amplifier in the light-receiving module is only several MHz. It is possible to provide accurate power control by using a configuration of the optical recording/reproducing apparatus with a relatively low cost.
For the DVD-rewritable media, it is desirable to perform the above-mentioned multi-pulse laser driving. If a sample/hold circuit is used to detect the peak power of the laser diode, the required bandwidth of the light receiving module and the subsequent processing circuits becomes very large, which will not be appropriate for practical use.
However, a sample/hold circuit may be used to detect the erase power of the laser diode when a long space data is recorded on the disk. By using this method, the detection of the erase power is possible.
Further, there is a method for controlling the bottom power or the peak power of the laser diode. In this method, in order to suitably control the bottom power or the peak power of the laser diode, a derivative efficiency of the laser diode may be initially calculated prior to the start of the recording process so that the current, derived from the calculated derivative efficiency, is added to or reduced from the bottom-level drive current used to produce the erase power, so as to obtain the peak-level drive current for the peak power of the laser diode.
The above-mentioned method is effective only when the derivative efficiency of the laser diode does not change and is maintained at a constant level. If the derivative efficiency varies, the error of the peak-level drive current obtained by using the above method will not be negligible.
As disclosed in Japanese Laid-Open Patent Application No. 9-171631, there is known an optical recording/reproducing apparatus that detects the peak-power optical output of the laser diode when it is driven at the peak-level drive current in a non-pulse condition. In the above-mentioned conventional apparatus, the peak-power laser beam when the laser diode is driven at the peak-level drive current in the non-pulse condition is detected, and the erase-power laser beam when a space is recorded on the disk is detected, and then the bottom-level drive current to the laser diode is corrected by using the detected peak power and the detected erase power. The laser diode is driven at the corrected bottom-level drive current to produce the bottom-level optical output.
Generally, it is necessary that the optical recording/reproducing apparatus always maintain the three recording power levels, including the peak power, the erase power and the bottom power for the laser diode, in order to obtain the optical waveform with good jitter characteristics when the data is reproduced from the phase-change recording medium.
However, when the above-described laser power control of the conventional apparatus is applied to the write pulse strategy for the DVD-rewritable media, there is a problem in that the formation of a mark on the recording layer of the disk when the laser diode is driven at the peak-level drive current in the non-pulse condition becomes deficient.